A single-vehicle collision last month involving a Tesla Semi electric truck took 50,000 gallons of water to extinguish and required aircraft to dump fire retardant overhead, according to a preliminary report on Friday from the National Transportation Safety Board.

The crash, which occurred on California’s Interstate 80 west of Lake Tahoe, is being investigated by the NTSB. CAL-Fire’s efforts put out the flames cooled the vehicle’s massive battery to keep it from reigniting and prevented the fire from spreading beyond the crash site, the NTSB said.

The Tesla truck, driven by an employee, was headed to the company’s battery factory in Sparks, Nevada, from a warehouse in Livermore, California, the report said. The incident closed down part of the I-80 for 15 hours.

Tesla CEO Elon Musk first showed off the Semi truck design at an event in November 2017, promising it would come to market in 2020. The company still hasn’t started producing the trucks in high volume, but it’s building out production lines at its Nevada facility.

“Preparation of Semi factory continues and is on track to begin production by end of 2025,” Tesla said in its second-quarter earnings report in July.

The NTSB report confirmed that Tesla’s driver assistance systems, which are marketed as Autopilot and Full Self-Driving (Supervised) in the U.S., were not “operational” at the time of the Semi collision and fire.

Tesla didn’t respond to a request for comment.

WATCH: The Tesla Semi is here